The present invention relates to a transflective liquid crystal display device, comprising a plurality of pixels, each comprising a liquid crystal layer, being sandwiched between front and back electrode means as well as front and back polarizer means.
The invention also relates to a method of generating a patterned xcex/4 (quarterwave) foil for use in a display as described above.
Due to its low power consumption, reliability and low price, liquid crystal displays, or LCDs have become the standard display choice for mobile applications, such as PDAs, laptops and cellular phones. However, the LCDs commonly used today have the disadvantages that they commonly exhibit low brightness, unsaturated colours, a limited viewing angle and/or low contrast. Consequently, it is expected that improved devices, such as active matrix reflective and transmissive LCDs will rapidly take over the market for mobile applications. Reflective LCDs are especially suited for outdoor use in direct sunlight. The contrast ratio is relatively low, compared with a transmissive display, and under poor illumination conditions, the brightness of this kind of display is low. On the other hand, transmissive LCDs have a good contrast ratio, but they become practically unreadable in direct sunlight illumination conditions. Furthermore, the transmissive display utilises a backlight, resulting in an increase of the power consumption.
Consequently, there is a need for a display having good display properties under all lighting circumstances. One solution is to use a so-called transflective LCD, which may be used in both a transmissive and reflective mode at the same time. The intensity of the backlight can thereby be tuned in order to fit the lighting conditions, either by hand, or automatically, using a photo diode or the like. This invention relates to an arrangement in a transflective liquid crystal display and a method for producing such a display.
The object of the present invention is to provide a transflective display having a high efficiency as well as an improved viewing angle dependency. A further object of the invention is to provide a transflective display having a high transmission for the bright state of the display.
These and other objects are achieved by a liquid crystal display device according to the introduction, being characterised in that an optical xcex/4 layer at least partly is arranged between said front polarizer and said liquid crystal layer, and said liquid crystal layer is a liquid crystal layer having a twist angle essentially within a range xc2x180-100xc2x0, such as 90xc2x0. By this arrangement, a transflective display having a high contrast ratio reflective mode may be achieved. This construction results in a device having a steeper reflection/transmission-voltage curve than prior-art reflective LCD devices with a lower twist angle, resulting in a reduced voltage swing on the column drivers, which in turn reduces the power consumption of the inventive display. Furthermore, it is less sensitive to cell gap variations in the transmissive mode. Preferably, said optical xcex/4 layer is a wide band xcex/4 layer, providing a display with a better overall dark state, having an improved contrast ratio and an increased brightness.
In accordance with a first embodiment of the invention, wherein each of said pixels are subdivided into a reflective and a transmissive sub-pixel, respectively, whereby said optical xcex/4 layer essentially only covers said reflective sub-pixels, thereby constituting a patterned xcex/4 foil. This display has a relatively high transmission. Preferably, a cell gap of a transmissive sub-pixel is essentially larger than a corresponding cell gap for a reflective sub-pixel. The cell gap of the transflective sub-pixel may for example be 1.5-2.5 times bigger than the cell gap for the reflective sub-pixel, and preferably around 2 times bigger. Thereby the backlight efficiency of the display may be further improved, since the larger cell gap of the transmissive sub-pixels results in an exit polarisation state with a smaller ellipticity for the polarised light, and thereby an increased transmission.
In accordance with a second embodiment of the invention said back electrode means is a semitransparent reflecting electrode essentially covering the entire pixel area, being easy to realise by adding standard components.
The above-described objects are also in part achieved by three different methods for generating a patterned xcex/4 foil for use in a display as described above.
In accordance with a first embodiment, the method comprises the following steps: depositing a reactive liquid crystal layer on a substrate, applying a mask, covering parts of the display corresponding to transmissive parts of the display, while revealing parts corresponding to reflective parts, and photo-polymerising said reactive liquid crystal layer, through said mask removing non-reacted liquid crystal material. This method has the advantage that the processing may be done at a single temperature, which reduces both the processing time and investments in equipment.
In accordance with a second embodiment, the method comprises the following steps: depositing a reactive liquid crystal layer on a substrate, applying a mask, covering parts of the display corresponding to transmissive parts of the display, while revealing parts corresponding to reflective parts, performing a first photo-polymerisation exposure of said reactive liquid crystal layer, while keeping the reactive liquid crystal layer at a first temperature, performing a second photo-polymerisation exposure of the reactive liquid crystal layer, while keeping the reactive liquid crystal layer at a second temperature, whereby one of said photo-polymerisation exposures are made through a mask, being applied on said reactive liquid crystal layer. This method has the advantage that LC material above the transmissive sub-pixel that has not reacted in the photo-polymerisation process described in said first embodiment, need not be removed. Preferably, said first and second temperatures is so chosen that the reactive liquid crystal layer is in a nematic liquid crystal phase at said first temperature, and at a temperature above a clearing point of said liquid crystal material.
In accordance with a third embodiment, the method comprises the following steps: depositing a reactive liquid crystal layer on a substrate, and providing a patterned orientation layer, corresponding to the desired patterned xcex/4 foil. The orientation of the xcex/4 foil above the transmissive sub-pixel is suitably parallel with either the transmissive or the absorbing axis of a front polariser. Preferably, said patterned orientation layer is generated by means of photo alignment. This method is advantageous in that photo-alignment as such is a rather simple and well-tested method. Furthermore, no mask is needed in the polymerisation of the reactive LC material.